Floral tributes to Jo Cox MP in Parliament Square. Parliament will be recalled on Monday to pay tribute to her.
Photograph: Hannah Mckay/EPA

Jo Cox was the victim of a “targeted” killing and detectives investigating the attack on the Labour MP are now treating the main suspect’s links to rightwing extremism as a “priority line of inquiry”.

The statement from West Yorkshire police on Friday came after the Guardian revealed that samples of Nazi regalia and far-right literature were found at the house of the suspect, Thomas Mair, 52.

A picture is now emerging of a deliberately targeted attack in which Mair lay in wait for the MP as she arrived for her constituency meeting on Thursday.

Police said the suspect had been declared fit to be interviewed after being medically examined and added that his mental health was also “a clear line of inquiry”. Sources said he was lucid when first questioned.

Dee Collins, the temporary chief constable of West Yorkshire police, said a murder investigation was under way into an “isolated but targeted attack” and added that the force had called on the specialist skills of counter-terrorism detectives.

Cox was stabbed and shot several times just before 1pm on Thursday as she arrived with two colleagues outside the public library in Birstall for a regular constituency surgery. She was declared dead at 1.48pm, leading to the halting of the EU referendum campaign and leading to an outpouring of grief across the country.

The Conservatives, Ukip and the Liberal Democrats all said they would not contest Cox’s vacant Batley and Spen seat in West Yorkshire.

Witnesses are understood to have confirmed that the attacker shouted “Britain first” or “Put Britain first” as he shot and stabbed the 41-year-old. Police said there was no indication that anybody else was involved in the attack, but they were investigating how the suspect came to have an unlawfully held firearm.

“All indications are that this is a single, isolated attack on Jo, for reasons that we are still trying to establish,” said Collins.

At the Joseph Priestley memorial in Birstall close to where Cox fell, Corbyn and David Cameron delivered emotional tributes.

The Labour leader said: “She was taken from us in an act of hatred, in a vile act that has killed her. It is an attack on democracy.”

He later said that “those who peddle hatred, terror and division” must be stopped from poisoning the UK’s national and political life.

Parliament would be recalled on Monday to pay tribute to the MP on behalf “of everybody who values democracy, free speech and the right of political expression”, Corbyn said.

Standing alongside Corbyn, Cameron said the nation was shocked at the death of one of parliament’s “most passionate and brilliant campaigners”.

The prime minister said it was time for the UK to think about the need to “treasure and value our democracy” and the fact that “members of parliament are out in the public, accountable to the public, available to the public, and that is how Jo died. She died doing her job”.

Flags were lowered to half-mast at Buckingham Palace, as well as the Palaces of Westminster and Holyroodhouse, and both referendum campaigns have suspended campaigning until Sunday as a mark of respect.

Doreen Lawrence, the Labour peer whose son Stephen was killed in 1993, said there were no words that could express what it felt like to have a young person whose “life is still full of possibility, brutally snatched away from you”. Writing in the Guardian, she said that the hopes and dreams of Cox’s family members had been “shredded in one foul afternoon”.

Lady Lawrence warned that a message of hatred against foreigners or people with different religions had been getting “louder” in the America and the UK, citing US presidential candidate Donald Trump. But she also hit out at comments by leading leave campaigner Boris Johnson about Barack Obama’s ancestry and said the poster unveiled by the Ukip leader, Nigel Farage, on Thursday was “reminiscent of Nazi propaganda”.

Vote Leave said it had cancelled events including a rally in Birmingham where Johnson was due to speak, while Britain Stronger In Europe said it was scrapping more than 2,000 events, including street stalls and a rally speech by Corbyn in Manchester.

Even when arguments resume, the referendum campaign is very unlikely to be fought at the same intensity as before, sources said. The Conservatives, Ukip and Liberal Democrats also said they would not contest Cox’s seat of Batley and Spen out of respect for the parliamentarian who won it in May 2015 with a majority of 6,057.

Meanwhile, it emerged that the suspect possessed far-right literature. According to the Southern Poverty Law Center, a US civil rights organisation, Mair bought books from a US-based neo-Nazi group, the National Alliance, including guides on how to build homemade guns and explosives.

He bought guides on the “chemistry of powder and explosives” and “incendiaries” and a handbook with instructions on constructing a pipe pistol using parts available in DIY stores, the organisation said. There were also reports that Mair was a subscriber to SA Patriot, a South African magazine published by White Rhino Club, a pro-apartheid group. The club describes the magazine’s editorial stance as being opposed to “multicultural societies” and “expansionist Islam”.

It was also reported on Friday night that Mair sought help for mental health problems at an alternative therapy centre close to the site of the attack in Birstall the night before Cox was killed. The owner of Birstall Wellbeing Centre, Rebecca Walker, said Mair became nervous as more people began to arrive and she told him to come back the following night and have a “cup of tea” and discuss his issues.

Walker said Mair told her that he had tried alternative therapies including reflexology and he was looking for one-on-one meditation classes because he was not a sociable person and he preferred small groups.

She said: “He said he was on medication for depression and told me he had been suffering from problems for a long time. He was calm but you could tell he was lonely and he was searching for some help. He started to get nervous when people began to arrive for the session and I told him to come back on Thursday night for some tea and a chat.”

Cox’s friends have set up a fundraising drive in her memory focusing on three charities, including Hope Not Hate which tackles extremist far-right politics in the UK.

It emerged that Cox was with her assistant Fazila Aswat as she died on Thursday. Aswat’s father, Ghulam Maniyar, told ITV News that his daughter held Cox in her arms as she bled.

“She said, ‘Jo get up’, but she [Jo] said, ‘No, my pain is too much, Fazila’, and I think those are the last words Jo spoke,” he said.

He said his daughter tried to hit the attacker with her handbag “but he tried to go at her … He came back again and shot her [Jo] again twice.”

Members of the public left bouquets on Cox’s houseboat on the river Thames where she lived with her husband Brendan Cox and two young children. More were laid in Parliament Square where Farage left a tribute which read: “A terrible waste of a life. Sincerest condolences to all the family.”

Former Labour leader Neil Kinnock was on the verge of tears as he described Cox as “a woman of huge intelligence, sparkling”.

In light of the killing, some politicians began to question the tone of the European Union referendum debate. Writing in the Guardian, the former prime minister, Gordon Brown, said Cox’s death “raises difficult questions … and we need to start answering in what remains of this referendum campaign”.

“The referendum was always about more than Europe,” he said. “It was always about what kind of Britain we are and what we aspire to be. And the tragedy is that the discourse of the referendum too easily descended from a vote on the future of Britain in Europe to a vote on immigration into Britain and then on immigrants and those who support immigrants. Unless we strive for a culture of respect to replace a culture which does too little to challenge prejudice, we will be learning nothing from what happened to Jo.”

Former Labour minister Yvette Cooper also criticised the “vitriol” in the EU referendum debate while the German chancellor, Angela Merkel, urged British politicians to moderate the language used in the campaign. “The exaggerations and radicalisation of part of the language do not help to foster an atmosphere of respect,” she said in response to a question about the killing.

Floral tributes are left close to where Jo Cox was killed in Birstall. Photograph: Matt Cardy/Getty Images

West Yorkshire police revealed that Cox had twice previously received “a malicious communication of a sexual nature at her parliamentary office in Westminster”. One resulted in an individual being arrested and cautioned in March.

Police forces are contacting MPs to “consider any new security concerns”, the National Police Chiefs’ Council said. Some were in their constituencies holding surgeries, having decided to proceed as normal, but others cancelled.

“I know MPs are scared,” said Dan Jarvis, the Labour MP for Barnsley Central. “We’ll be reviewing our security, but I’ll walk through Barnsley today like every Friday.”

Labour MP Rachel Reeves said: “The work of an MP in our surgeries, our work in the community, must continue. But I think it’s right today that, as well as ceasing the campaigning in the referendum, we close our office.”

Cox was widely praised for her work in parliament for refugees, particularly those from Syria.

Last Friday, she gave a talk to children at Whitcliffe Mount school in her constituency entitled: “What are immigrants and migrants? What’s the difference”. A week later the school stopped to remember a regular visitor who “inspired us all to … be compassionate to those less fortunate than ourselves”.